The LED display device of Patent reference 1 has a panel of plural LEDs (hereinafter to be referred to as light-emitting diodes) arranged in an array configuration, a common driver connected to the plural light-emitting diodes by line wires, and a constant-current driver (constant current source) connected to the plural light-emitting diodes by means of row lines (see FIG. 1).
A power supply voltage is fed from the common driver to light-emitting diodes selected by means of the line wires and row lines, and a preset constant current flows by means of the constant current driver.
The luminance of a light-emitting diode with the voltage applied to it can be controlled by adjusting either the magnitude (amplitude) of the current flowing in the light-emitting diode or the power ON time.
Consequently, by feeding power corresponding to the duty to a light-emitting diode connected to the constant current source, the light-emitting diode can emit light at an appropriate luminance corresponding to the duty.
For a light-emitting diode, when a current flows in the forward direction from the anode to the cathode, light is emitted. In this case, a fall in voltage takes place in the forward direction of the light-emitting diode. Consequently, the light-emitting diode emits light at a luminance corresponding to the amplitude of the current or the power ON time. Consequently, it is necessary to apply a voltage to the light-emitting diode higher than the fall in voltage in the forward direction of the light-emitting diode.
When plural light-emitting diodes are connected in series, it is necessary to apply a voltage to the plural light-emitting diodes higher than the sum of the forward drops in voltage generated in the forward direction of the plural light-emitting diodes.
The fall in voltage of a light-emitting diode is different for different elements continuously manufactured due to dispersion in manufacturing. The temperature characteristics of a light-emitting diode are also different for different elements actually manufactured.
Consequently, the fall in voltage in the forward direction is smaller for some light-emitting diodes, and it is larger for some light-emitting diodes.
As a result, for an actually manufactured LED display device, it is assumed that the fall in voltage in the forward direction is maximum, and a power supply voltage higher than the assumed voltage is fed from the common driver to the light-emitting diode.
On the other hand, for an actually manufactured LED display device, the fall in voltage in the forward direction of the light-emitting diode may be smaller than that assumed voltage.
Here, if the light-emitting diode actually used has a small fall in voltage in the forward direction of the light-emitting diode, an excessively high voltage is applied on the light-emitting diode.
Also, due to the excessively high voltage, excess power is consumed and excess heat is generated in the LED display device.
The following listed equation 1 is for computing the excess power. In equation 1, P(Loss) represents the excessively consumed power, Vf(max) represents the maximum value of the fall in voltage in the forward direction of the light-emitting diode due to dispersion in manufacturing, Vf(min) represents the minimum value of the fall in voltage in the forward direction of the light-emitting diode, N represents the number of light-emitting diodes connected in series, and I represents the current flowing in the light-emitting diodes.P(Loss)=(Vf(max)−Vf(min))×N×1  Equation 1